1. Field of the Invention
This invention relates to a semiconductor device with a contact structure for connecting a conductive region formed on at least one of a junction formed in and a gate insulating film formed on a substrate, such as a diffusion region and a gate electrode of the semiconductor device, to a metal wiring. This invention also relates to a method of manufacturing a semiconductor device including such a contact structure.
2. Description of the Related Art
In a semiconductor device including MOSFET transistor, source and drain regions of these transistors are connected to metal wirings by means of contact structures. In a known semiconductor device, a contact structure is formed by the following steps. At first, an insulating film such as an interlayer insulating film, i.e., a SiO.sub.2 film, is formed on a silicon substrate having a surface in which transistor regions are formed. Then, the insulating film is selectively removed to form a contact hole. Next, a Ti film, a TiN film and an Al film are successively deposited by sputtering on the surface of the insulating film as well as in the contact hole. After that, a multilayered metal film on the insulating film is selectively removed in accordance with a desired pattern to form metal wirings.
In this known contact structure, the Ti film reacts with silicon atoms of the diffusion region during a heating process after the deposition to form a silicide which serves to decrease a contact resistance between the diffusion region and the metal wiring. The TiN film serves to prevent the mutual movement of the silicon atoms and metal atoms of the metal wiring, so that a junction formed between the diffusion region and a bulk of the silicon substrate or a well formed in the surface of the silicon substrate can be protected against breakage. That is, the TiN film serves as an anti-diffusion film. The Al film constitutes a main body of the metal wiring and serves to conduct an electric current at a low resistance.
However, in the usual sputtering method, it is rather difficult to deposit an aluminum alloy film within a fine contact hole. Therefore, when manufacturing a semiconductor device including contact holes having a diameter smaller than about 0.8 .mu.m, a metal film is not deposited simultaneously on the insulating film as well as in the contact holes. Rather, metal plugs are first formed within the contact holes. Then, a metal wiring film is deposited on the insulating film as well as on the plugs.
Conventionally, the source and drain regions of MOS transistors are formed by diffusion regions having impurities such as A and B diffused therein. In recent MOS transistors, it has been required to make a gate length shorter than about 0.5 .mu.m. In this situation, the resistance of the diffusion regions becomes comparable to a channel resistance. Therefore, particularly in high speed logic integrated circuits, a silicide of a high melting point metal, such as TiSi.sub.2, is formed on a diffusion region in order to decrease the resistance of the source and drain regions.
FIGS. 1A to 1D are cross-sectional views showing successive steps for manufacturing a MOS transistor in which a silicide of high melting point metal is used.
As shown in FIG. 1A, wells 3 are formed on a surface of a silicon substrate or wafer 1 such that they are isolated by the field oxide film 2. A MOS FET is formed in a well 3. A gate of the MOS FET is formed by a gate insulating film 4, such as oxide and nitrided oxide films, a highly doped polysilicon layer 5 and a silicide film 6, such as TiSi.sub.2. On both sides of the gate, a source region 7 and a drain region 8 are formed. Silicide films 9 and 10 are formed on the surfaces of the source region 7 and drain region 8. The silicide films 6, 9 and 10 may be formed by a known salicide method which is described in, for instance, C. Y. Ting et al, 2nd International IEEE VLSI Multilevel interconnection Conference, (1985) p. 307.
Next, as illustrated in FIG. 1B, an insulating layer 11 is formed on the silicon substrate 1. A contact hole 12 is formed in the insulating layer 11 by selectively removing a portion of the insulating layer 11. Then, a metal composite film 13 is deposited on the insulating layer 11 as well as on an inner surface of the contact hole 12 by depositing thin Ti film and TiN films successively in this order. Then, a tungsten (W) film 14 is deposited on the whole surface of the substrate by chemical vapor deposition (CVD) using a hexafluoro tungsten (WF.sub.6) gas as a raw material gas as shown in FIG. 1C. During this CVD process, the metal film 13 serves as an adhering film. Then, the tungsten film 14 is etched back to form the plug 15 embedded in the contact hole 12, as shown in FIG. 1D. Finally, a metal film is formed on the insulating film 11 such that it is connected to the tungsten plug 15. The metal film is then patterned into a desired metal wiring.
In the known method of manufacturing the contact structure including the plug, after the Ti/TiN film 13 is formed on the insulating film 11, the tungsten film 14 is deposited on the insulating film while the Ti/TiN film is used as the adhering film. Then, the tungsten film 14 is etched back to form the plug 15 within the contact hole 12. It is apparent that this known method is rather complicated.
Further, if the contact hole 12 is formed with a reversed taper shape, during the formation of the tungsten film 14, a seam or void is liable to be formed within the contact hole 12. Then, it is no longer possible to obtain good electrical contact and the reliability of the MOSFET is decreased. Moreover, when the tungsten film is formed from the WF.sub.6 gas, it contains a large amount of fluorine. It should be noted that fluorine is very corrosive, so that the metal wiring is damaged and the transistor characteristics are degraded.
It has further been proposed to form a plug within the contact hole by means of selective CVD, as shown in FIG. 2. In this selective CVD, a WF.sub.6 gas flows along the surface of the substrate and a tungsten film is selectively deposited in the contact hole 12 where the titanium silicide film 10 is exposed at the bottom of the contact hole. In this method, the contact hole 12 can be completely filled with tungsten and no seam or void is formed in the tungsten film.
However, even in this method, a large amount of fluorine is incorporated into the tungsten plug 15. Moreover, the fluoro series gas flowing along the surface of the substrate is reacts with the titanium silicide film 10 to form particles of TiF.sub.3 16 on the surf ace of the titanium silicide film 10. These TiF.sub.3 particles 16 at a boundary between the plug 15 and the silicide film 10 increase the contact resistance.
In order to mitigate the above mentioned drawback, it has been also proposed to perform selective CVD of tungsten after converting the surface of the titanium silicide film 10, which is exposed at the contact hole 12, into a TiN film by using a nitriding treatment. This method has been described in "VLSI Technology Symposium", Martin S. Wang et al, 1991, p. 41. However, this method does not solve the problem of the corrosion and degradation of property due to fluorine.
In Japanese Patent Application Laid-open Publication 3-110838, another method of forming a contact plug is proposed, by selective CVD using a raw material gas containing aluminum. A similar method has been also described by Kouichi Tani and Satoshi Nishikawa (OKI), in Extended Abstracts 1993, International Conference of SSDM, p. 543 (1993). In these publications, an aluminum plug is formed directly on the surface of the silicon substrate. However, as shown by Tani and Nishikawa, the surface of the silicon substrate is eroded by the aluminum during CVD and the junction characteristics are degraded. Until now, no publication about the formation of aluminum plug by selective CVD on a silicide film formed by the salicide method is available.
In selective CVD of aluminum, the aluminum has to be deposited only in the contact hole and not on the insulating film. However, in practice, the aluminum is liable to be deposited on the insulating film as well as on the side walls of the contact hole, due to the dangling bonds existing on the surface of the insulating film. Therefore, a void might be formed within the aluminum plug. Moreover, the aluminum deposited on the insulating film might short-circuit the metal wirings.
It has been further proposed to form the TiSi.sub.2 film by means of CVD using TiCl.sub.4 gas and SiH.sub.4 gas. However, this CVD process is difficult to control. In addition, silicon atoms might be sucked from the diffusion layer into the TiSi.sub.x film so that the contact property is deteriorated. At the same time, an excess amount of silicon atoms supplied from the SiH.sub.4 gas remain within the TiSi.sub.x film, so that the contact resistance increases. Particularly, when a thick film is to be deposited, it is very difficult tog avoid these drawbacks, so that the contact hole cannot actually be filled only with the TiSi.sub.2 film.
It has been further proposed to heat the substrate at a temperature above 450.degree. C. during the deposition of the aluminum alloy film by sputtering, such that aluminum alloy flows into the contact hole. This method is disclosed in "T. Hariu et al, 1989 International Reliability Physics Symposium", p. 200, and Chen et al, 1990 International Electron Device Meeting, p. 51. However, this known method also requires the Ti/TiN metal film in order to make the aluminum alloy film flow into the contact hole, as in the method in which the plug is formed by depositing the tungsten film all over the substrate surface. The Ti/TiN metal film is also required to decrease the contact resistance and to avoid the degradation of the junction due to the temperatures during the deposition of the aluminum alloy film.
Moreover, this method can be effectively applied only to contact holes having a diameter larger than about 0.8 .mu.m. If the contact hole has a smaller diameter than 0.8 .mu.m, it requires further complicated processes, such as the deposition of a Ti film on the TiN film prior to the deposition of the aluminum alloy metal to increase the ability of the aluminum alloy film to flow into the contact hole. In this manner, this known method requires a rather complicated process and cannot be actually used in production scales.